Electronic switches of a known type, actuated by manual finger pressure or other such engagement, employ a piezoelectric element to generate a switching voltage for an electronic switching circuit. Such electronic switches have a wide variety of applications. A particularly important application is in the manually actuated switches for elevators in public buildings, namely the so-called hall call switches and car call switches.
In the prior art, it is a common practice to construct momentary push button switches with a thin plate of piezoelectric material, preferably a ceramic plate, which is operated in a bending mode by finger pressure to generate a signal voltage for an electronic switching circuit. Typically, the piezoelectric plate is bonded in face-to-face engagement with a thin metal plate so as to form a bendable laminate responsive to finger pressure. In this prior art, the push button overlays the laminate and imparts pressure to the face of the metal plate urging it in a direction toward the piezoelectric plate tending to bend the piezoelectric plate so that its free surface is convex. This generates a DC voltage between a first electrode engaging the free face of the piezoelectric plate and the metal plate which serves as a second electrode on the other face of the piezoelectric plate. The polarity of the generated voltage in this push button operation is such that the voltage on the first electrode is positive relative to that on the second electrode and is applied to the electronic switching circuit to cause a change of switch state, i.e. from open to close.
It has been found that such prior art piezoelectric transducers are temperature responsive to such an extent that certain temperature conditions may cause undesired switch operation. Further, it has been found that the laminate formed by the piezoelectric plate and the metal plate responds to increasing temperature by bending of the piezoelectric plate such that its free face is convex and hence it generates a signal voltage of the same polarity as that produced by finger pressure. If the rate of change of temperature rise is fast enough to overcome the voltage bleed-off effects in the switching circuit, the switch will turn on. In case of a sudden heat rise, caused for example by a building fire, the switch may be inadvertently actuated by the differential thermal expansion in the laminate. It is believed, for example, that in a public building elevator system sudden heat rise caused by outbreak of fire on one of the floors could result in a dangerous situation by calling elevator cars to that floor. Also, it has been found that such prior art electronic switching circuits are susceptible to malfunction as a result of a prolonged high temperature exposure or "soak"; such a high temperature soak will, at some point, cause the electronic elements of the switching circuit to leak or conduct. When the leakage gets high enough, the output switch, such as a field effect resistor (FET) will turn on.
A general object of this invention is to provide an improved electronic switch of the piezoelectric type and overcome certain of the disadvantages of the prior art.